1. Field of the Invention
The present invention relates to an axial valve element having a spherical surface on its tip end portion and more particularly to a method of manufacturing the valve element with the spherical surface.
2. Description of Related Art
Conventionally, there has been known, as a valve element applied to a hydraulic valve, an axial valve element having a spherical surface on the tip end portion thereof. Moving the spherical surface close to or away from a seat allows the opening of the hydraulic valve to be adjusted and the flow rate of liquid flowing through the valve to be changed. In this case, if the valve element is brought into contact with the seat and the tip-end spherical surface of the valve does not match the seat, then leakage of fluid may occur. For that reason, the spherical surface requires strict molding accuracy.
Meanwhile, to obtain a valve element with a spherical surface as stated above, several manufacturing methods have been proposed. The first method is, for example, to prepare a steel ball in advance and embed it in a recessed portion formed on the tip end portion of an axial valve element. Although a valve element having a highly accurate spherical surface is provided, this method requires preparing the material of the axial valve element and the steel ball separately and sophisticated technique for embedding the steel ball. This results in an increase in manufacturing costs.
It is possible to shave the tip end portion of the axial valve element into a spherical surface with a turning tool so as to make manufacturing process relatively easy. With this method, however, shavings remain on the spherical surface. As a result, the accuracy of the spherical surface tends to be lowered.
When it comes to an easy molding method, meanwhile, a cold forging method is also proposed. The method is to compress and then plastically deform the material of an axial valve element with a die in an ordinary temperature. FIGS. 16 through 19, which are an example of the valve element, shows how a spherical surface is molded on the tip end portion of the material by cold forging. FIG. 16 shows the tip of a material 41. Specifically, FIG. 16 shows a state in which the tip end portion 41a of the material 41 has been cut simply in perpendicular direction. FIGS. 17 and 18 show the process in which a special flange 42 is molded on the axis of the material 41. Here, a special die is used for molding the flange 42. FIG. 19 shows a state after the spherical surface has been molded on the tip end portion 41a of the material 41. In this case, the tip end of the material 41 is compressed within a die 44 having a semi-spherical, recessed surface 43 on its tip end portion, as shown in FIG. 20.
According to the cold-forging molding method stated above, the grinding undercut of the fill on the periphery of the tip end portion 41a of the material 41 remains at the time of die molding and tends to interfere with plastic deformation. Due to this, as shown in FIG. 20, molding is forced to be finished incompletely before all parts of the tip end portion 41a of the material 41 are press-welded to the semi-spherical, recessed surface 43. That is to say, the fill on the periphery of the tip end portion 41a remains incompletely deformed on the tip end along the semi-spherical, recessed surface 43. In this way, the tip end portion 41a of the material 41 could not sufficiently be formed by the conventional and ordinal cold forging.